As goes the paths of life, my game group's opportunity to get together is not near as regular as it was in the past. We still want to move through our epic adventure path, but I'm trying to find ways to speed up combat, which is always that bit that slows game night down - particular with massive combat that happens regularly.

I've implemented some means to speed things up, e.g., troops & squads, static damage for adversaries, limiting use of maps & minis

I've thought about another option: glass jaws. Basically, keeping the NPC's offensive the same, but halving hps to take them out more quickly. The question I have is how to set the CR. Any suggestions?

Also, if you have any other suggestions on expediting combat, I'm open to ideas.

I would use the table in the dmg for this since it's a fairly simple hp change.

As a rule of thumb, halving hp halves xp value.

Would this be the case even if all other stats were left alone?

Quote from Saurstalk >>

Would this be the case even if all other stats were left alone?

 The basic logic behind adjusted xp value is that xp value is proportional to the resources (including hit points) that need to be spent to overcome the threat. In general a monster that lives half as long is going to do half as much damage, ergo half the xp value (and doubling survival time, such as mythic monsters, should double xp value).

Makes sense to me.  Thanks for the explanation.

Which table would that be?

Quote from Saurstalk >>

Which table would that be?

 There's a table for designing monsters in the DMG, but it's quite inaccurate for modeling actual published monsters.

If your idea is to speed game play because of lack of time then might I suggest you don't worry about it. 

If you half the CR, and I would imagine the xp reward, your also doubling the amount of combats you'll need to run, for your players to level. That will lengthen your game. Instead let them steam roll some combats, and gain the normal xp. Then when you have a major battle useing normal hp, or even max hp for that battle it will by contrast feel like one hell of a battle.

If you want to speed up combat without just making everything super easy, I suggest reducing hit points and increasing damage; a monster that has half as many hit points and does twice as much damage is generally about as dangerous as a regular monster (subject to quirks with attacks that have a recharge).

If you are looking at ways to speed up combat by making your NPC easily defeatable, just have everyone narrate what they want done and forego all the dice rolling. I can understand, and frequently use the Dials of Monster Difficulty when I may have been heavy handed in setting up the encounter, or it seems that the PCs will obliterate their targets without any meaningful struggle. I don't suggest that you bother with altering the CR and XP of the monsters that you use. You'll spend more time doing that than running combat. I can also suggest adding 4e-style minions to boss battles, or as a group of easily defeated baddies, but that won't speed up combat. 

Some things that I've used to speed up combat:

• Use side initiative vs. individual initiative.
• Group your monsters by type and run them as a mob with mob rules.
• Use the "PC1 you're up, PC2, you're on deck" method of initiative callout so that everyone is ready to go on their turn.
• Coach your players to have their actions ready on when it is their turn (this saves the most time, IMHO). 
• Everybody rolling their to-hit and damage dice at the same time.
• Letting the players know what the AC of the monster they are facing is so as to remove the back and forth of "Does a 17 hit?","Yes, roll damage", "6pts of throwin it back damage". If they know the AC of the mob, they can tell you if its a hit or not, and how much damage it takes. (And before anybody decides to light the pilot on their flamethrower, admit that your players will eventually narrow down what hits and what doesn't. If the goal is speed, remove the governor. Also, this method means that you have to trust your players to adhere to a social contract of no cheating dice rolls, and not allowing bleedthrough to the PC's actions)

I'd not consider halving HP of a creature to half it's CR. Regarding the resource expenditure, it's also for surviving an enemy, not just killing it. On their turn, it doesn't matter if a goblin has full health, 1 health, or a million temporary HP - it hits just as hard. Putting twice as many in will double the damage output.

I would consider the following:

1: Shift to milestone leveling, that way the story will progress depending on progress, not on how many things died along the way.

2: Be willing to add damage for good rolls. If a player rolls a crit, and it deals nearly maximum damage, and would leave 5hp on the enemy - just kill the enemy. Don't leave the dregs, it'll only slow down a combat.

3: Be willing to narrate from the tipping point. The first flush of combat is the fun bit - walking around in initiative order killing the last few enemies clinging to 1-3hp is just a chore. Cut out the chores - don't leave enemies with insignificant HP (by all means, have them fall down but not be dead, just holding wounds, for example, but get them out of the fight.), don't make the players go through the motions when they've clearly won. Run it by your players before you start doing this, but you can speed things up by saying "Ok everyone, the enemy are fairly well defeated and starting to run off - do you want to chase them, or hold here, or anything else?". 

4: Remember, if the players are having fun, then your game is going well. Some things take time!

CR is already pretty messy and imprecise. If you're tampering with the formula this much you may as well just drop the concept altogether and use your own threat gauge based on what the party has handled in the past.

Halving hp is not going to be a linear decrease in challenge. A party spreading their damage around to get every enemy to half hp in the first round is still going to take a full round of attacks. The same salvo on a glass jaw version of the enemy will completely wipe them out. Many half hp monsters will be killable by one PC in one turn, and stuff that activates on kills like Great Weapon Master will create a positive feedback loop that leads to an absolute steamrolling.

Likewise, doubling monster damage to keep the threat high is going to make combat very swingy. If Team Monster gets lucky on initiative, they could easily drop a couple PCs before they ever get to go. If the PCs go first, they may never get touched. It begins to minimize the importance of player choice and tactics and just becomes a race to see who strikes first.

tl;dr 5e is already designed for combat to be as quick as possible while still leaving enough time to showcase player/monster abilities. If you mess with it this much, you will get unintended side effects. There are servers full of digital ink across the internet detailing better ways to speed up combat without tampering with the basic math.